1. Field of the Invention
The present invention relates to a zoom lens camera having at least two front and rear groups of lenses and more particularly it relates to an apparatus for focus adjustment called a back adjustment (back focusing).
2. Description of Related Art
A zoom lens camera has at least two groups of lenses, i.e., a front group of lenses and a rear group of lenses, so that the groups of lenses are moved together or separately to control the spatial distance therebtween to allow zooming. Generally, a cam ring is used to control the position of the groups of lenses, so that when the cam ring is rotated, the lenses are moved to control the spatial distance therebetween.
In a conventional zoom lens camera, it is necessary to provide zooming adjustment for minimizing the displacement of the focal point at a desired focal length and a back adjustment for making the focal point coincide with the position of the film after the zooming adjustment is completed.
This zooming adjustment is usually effected by adjusting a relative position of a lens ring which supports the groups of lenses on a cam ring follower. The lens ring is screw-engaged by the cam ring follower which is moved in accordance with the rotation of the cam ring and accordingly the zooming adjustment can be effected by the adjustment of an axial position of the cam ring.
However, since the back adjustment is an adjustment of the axial position of the whole of the groups of lenses after the zooming adjustment is completed to focus an image precisely onto a film plane, it is necessary to adjust the axial position of the cam ring. Therefore, in a conventional camera, an immovable cylinder (lens unit) which rotatably supports the cam ring while preventing the axial displacement thereof is secured to a camera body through a washer which is inserted between the camera body and the immovable cylinder in accordance with a premeasured distance between a reference surface of the lens unit which comes into contact with a reference surface of the camera body and a focal point, so that the focal point meets the film plane.
However, in the back adjustment mechanism as mentioned above, it is not only necessary to determine the measurement of the focal point for each lens unit upon assembly of the camera, but also to prepare many kinds of washers corresponding to the measurements, resulting in difficulty in diassembly. Furthermore, if the dimensional precision of washers or the associated elements is out of allowable limits, it is impossible to perform the back adjustment by using the washers. In order to make a readjustment, it is necessary to disassemble and reassemble substantial parts of the camera because of the inside location of the washer(s), resulting in inefficiency.